1. Field of the Invention
The present invention relates to a wet type friction material which is used in a clutch facing of a power transmission system of an automobile, an industrial machine, a railroad vehicle, etc., and which is used in an oil.
2. Description of the Related Art
Concerning friction materials for wet type clutch facings, a variety of chartaceous friction materials have been used widely. A chartaceous friction material comprises a substrate and a binder which is impregnated into and cured in the substrate. The substrate is formed of organic fibers and a several types of friction conditioners admixed with the organic fibers. The binder is composed of a thermosetting resin, such as a phenolic resin, etc.
However, the aforementioned friction material has a problem in that it exhibits not only a low heat resistance but also a low friction coefficient. Accordingly, a variety of countermeasures are taken to solve the problem.
Regarding one of the countermeasures, a number of friction plates is increased to enlarge the contact area with respect to a mating member, for instance, in a field of clutch facings. However, the construction of the wet type clutch is not only complicated but also large-sized by adopting such a countermeasure. Accordingly, the counter measure results in a large energy loss and a high cost.
Moreover, a trial has been carried out in which a copper-based sintered friction material is applied to a wet type clutch. If such a sintered friction material can be utilized, even when the number of friction plates is less and the contact area is small, the wet type clutch can satisfy the heat resistance and the strength against pressure. Consequently, the aforementioned problem can be solved. However, many of the sintered-metal-based friction materials exhibit a friction coefficient lower than organic friction materials in most of the cases. Hence, as a wet type clutch for a recent high-performance automobile, the sintered-metal-based friction materials cannot necessarily exhibit satisfactory friction characteristics.
Japanese Examined Patent Publication (KOKOKU) No. 7-021,298 discloses a two-layered wet type friction material which comprises a friction-surface side and a fixed-surface side. The friction-surface side includes a chartaceous substrate into which a thermosetting resin is impregnated. The chartaceous substrate is prepared by a papermaking machine with fibrous components, inorganic fillers and friction conditioners in a wet manner. The fixed-surface side includes a chartaceous substrate into which a thermosetting resin is impregnated. The chartaceous substrate is prepared by a papermaking machine with fibrous components in a wet manner. However, in the friction material, the fixed-surface side contributes only to improving the cooling ability and the mechanical strength, but little affects the friction characteristics. The friction characteristics depend on the friction-surface side itself. The friction-surface side exhibits a friction coefficient which is equivalent to those general wet type friction materials.